Some personal computers (PC) employ a hard disk device as a secondary storage device. In such PCs, a technology is known for backing up data that has been stored in the hard disk device to prevent the data from becoming invalid because of some failure. For example, when act of changing data in the hard disk device is detected, a snapshot as a backup copy of the data before the change is taken and a log of changes made to the data is generated. Then, processing for taking a new snapshot, invalidating a log taken in the past before the new snapshot was taken, and generating a new log is repeated at every predetermined time (see, for example, US Patent Application Publication No. 2006/0224636). In case data becomes invalid due to some reason, the data can be restored by referring to the snapshot and the log. There are typically two types of logs: pre-log and post-log. The pre-log is a log that is generated, before update of data, as an update plan indicating how the data is going to be updated. On the other hand, the post-log is a log for approving, after the data is actually updated, updating of the data.
In recent years, a capacity of a NAND flash memory as a nonvolatile semiconductor storage device has been increased dramatically. As a result, PCs including a memory system having the NAND flash memory as a secondary storage device have been put to practical use. For example, in the NAND flash memory, data writing and readout are performed in page size units and data erasing is performed in block size units. Therefore, in the NAND flash memory, the pre-log and the post-log are written in various pages of various blocks, respectively. A technique for storing a plurality of data (multi-value data) equal to or larger than 2 bits in one memory cell (a multi-value memory technology) is sometimes employed to the NAND flash memory (see, for example, JP-A 2004-192789 (KOKAI)). Therefore, in PCs that incorporate a NAND flash memory, processing for resetting the PC from abnormal isolation or the like of a power supply is difficult or data cannot be restored.